Abscisic acid (ABA) is a phytohormone that plays an essential regulatory role for a variety of physiological processes. The phytohormone is involved in embryo development, seed dormancy, transpiration, and adaptation to environmental stresses. ABA regulates many agronomically important aspects of plant development including synthesis of seed storage proteins and lipids as well as regulating stomatal closure. The analysis of ABA-responsive promoters has revealed a diversity of potential cis-acting regulatory elements.
Mutations in ABA biosynthesis are known in a variety of plant species. See, for example, Leung and Giraudat (1998) Annu. Rev. Plant Physiol. Plant Mol. Biol. 49:199-222, and the references cited therein. In Arabidopsis, a number of genetically distinct Arabidopsis acid-insensitive loci have been identified. These mutants were selected based on the ability of seeds to germinate in the presence of inhibitory concentrations of ABA. The mutations have also been shown to affect several additional aspects of seed development, including accumulation of storage proteins and lipids, chlorophyll breakdown, and desiccation tolerance.
To date, numerous mutants and genes have been characterized in plants. Five mutationally identified ABA response loci have been cloned. These represent three classes of proteins. The classes include two orthologous transcriptional regulators (Viviparousl-Vp1) of maize and ABA-insensitive 3 of Arabidopsis (ABI3), two highly homologous members of the protein phosphotase 2C family, and a farnesyl transferase of Arabidopsis. See, for example, McCarty et al. (1991) Cell 66:895-905; Giraudat et al. (1992) Plant Cell 4:1251-1261; Leung et al. (1994) Science 264:1448-1452; Leung et al. (1997) Plant Cell 9:759-771; and Cuither et al. (1996) Science 273:1239-1241.
During the maturation phase of seed development, the embryo becomes quiescent in tissues that are destined to remain viable and the dry seed acquire tolerance to desiccation. In maize and other grasses, this includes cells in the aleurone layer of the seed endosperm. The viviparous mutants of maize are blocked in the maturation program. Thus, the mutant embryo proceeds precociously into seedling development while attached to the mother plant. The nine characterized vivipary loci affect early steps in the biosynthesis of carotenoids and abscisic acid. vp1 embryos exhibit reduced sensitivity to ABA in culture. It has been suggested that the initial Vp1 may encode a factor involved in ABA perception.
At the molecular level, embryonic maturation is associated with a broad range of gene activation. Many of the genes expressed are regulated by the hormone ABA. However, the molecular mechanisms of ABA action are largely unknown.
ABA mediated growth control is a fundamental response of plants to adverse environmental conditions. Because little is known about the molecular mechanism of ABA-mediated growth control, methods are needed to modulate the response of plants to ABA, particularly to increase yield.